1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus equipped with the developing device.
2. Description of the Related Art
In a traditional image forming apparatus, the following procedures are carried out: an electrostatic latent image is formed on a photosensitive surface of a photosensitive drum as an image carrier followed by developing the latent image to be made visible on the photosensitive surface as a toner image by a developing device using a developer containing toner therein. Subsequently, the toner image is transferred to a paper, and that transferred toner image is fixed on the paper.
FIG. 6 is schematic diagram illustrating an internal configuration of the traditional image forming apparatus.
As shown in FIG. 6, there are disposed in proximity to the circumference of a photosensitive drum 103: an electrical charger 104; an exposure portion (not shown); a developing device 106J; a transfer portion 111; a fresh toner supply portion 115; and a cleaner portion 116.
The developing device 106J is disposed downstream of an irradiation position along a moving direction of the photosensitive surface of the photosensitive drum 103 and accommodates a two-component developer (Hereinafter, referred to simply as a “developer”) in a developer vessel 20 so as to supply the developer to the photosensitive surface of the photosensitive drum 103 by a developing roller 27. In this manner, an electrostatic latent image formed on the photosensitive surface of the photosensitive drum 103 is made visible for forming the toner image thereon.
More specifically, the developing roller 27 is configured to hold the developer on its surface (or a roller surface) by magnetic force. The magnetic force is distributed on the roller surface so as to be strong at a position (i.e., in the vicinity of a developing position confronting the photosensitive drum 103) where the retention of the developer is required and so as to be weak at a position (a so-called “recovery pole”) where the developer that is residual on the roller surface after a developing process should be disengaged from the roller surface.
With this configuration, the developer residual (or remaining) on the roller surface of the developing roller 27 after the developing process is dropped downwardly by gravity as it moves to approach the position at which the magnetic force on the roller surface is weak. The resultant dropped developer is again supplied to the developer roller 27 as being agitated by a first mixer 24a. 
In the developing device thus described above, the toner tends to be deposited within the developer vessel 20, especially on a position which is located or provided on an inner wall part which is located in the developer vessel and which confronts the roller surface of the developing roller 27 and where the toner begins to drop from the developing roller 27 during rotation thereof. Thus, when the toner is deposited in a certain amount and subjected to various stresses, a toner-clump is often generated due to such a deposited toner.
When such a toner-clump being subjected to vibrations during the loading or unloading of the developing device, the toner-clump is dropped from its deposition position into normal developer which does not contain any toner-clump, and mixed with each other. If the toner-clump is transported onto the photosensitive surface of the photosensitive drum 103 by the developing roller 27, it disadvantageously results in deterioration in picture quality (a so-called “clump-fog”) on the image formed on the paper sheet material. That is, it is believable that the toner-clump contributes to this deterioration.